The conventional way to provide fail-safe flame monitoring is to employ a mechanical shutter which interrupts the passage of light from the flame to a photocell at a predetermined rate of interruption. In this way, a "flame-on" condition will be indicated only by signals from the photocell occurring at the predetermined rate. Any other response from the photocell will indicate either a "flame out" condition or an equipment failure, and, in order to be safe, the burner system will be shut down to determine what the problem is. This is what is meant by the term "fail-safe".
Mechanical shutters, however, have numerous drawbacks. They have a motor drive and moving parts, which can give trouble. The bearings also are a problem, especially in the portions nearest the flame where the heat is greatest and lubrication is difficult. They take up space, they are labor-intensive, and, therefore, expensive to install. All of these factors contribute to their high initial cost and high cost of maintenance. It is, therefore, a general objective of this invention to provide a flame monitoring equipment, which is fail-safe but which eliminates the mechanical shutter.
Another problem encountered in modern flame monitoring, is the detection of flames in the presence of smoke, pulverized coal, dirt, ash or other adverse condition which may be associated with the flame in a fire-box. In this connection, a high degree of sensitivity is desireable along with full fail-safeness. Accordingly, the provision of high sensitivity together with full fail-safeness is a further object of the invention.